A. Technical Field
This invention relates generally to optical transport networks, and more particularly to the management of skew across a wave division multiplexed network.
B. Background of the Invention
Optical networks are able to communicate information at high data rates. The importance of maintaining the relative timing and sequence of this information being communicated is important for proper operation of such networks.
A conventional optical transport system 10 is shown in FIG. 1, with multiple intermediate nodes and routes 16 between source 12 and destination 14. Nodes n1-n6 (lower case) in a network are provided as an exemplary network with spatial diversity in the span, or segments separating nodes, e.g., across a geographic area. Multiple communication paths between a source node and destination node are provided across the network. The transport system might consider the route length, the traffic load, the route cost, and latency property, among other factors, for a given signal when choosing a path within the network on which to transport the signal. For example, a high quality of service (“QoS”) request might require a given signal be transported on a route between a source and a destination with the lowest amount of latency. Alternatively, as traffic data rates continue to mushroom, carriers typically resort to routing signals on alternative and/or relatively higher latency paths, which oftentimes span a longer overall distance than the preferred path. Additionally, these longer paths typically have more nodes, which usually translates into compromised timing properties for the signal at the receiver.
Certain optical networks may transmit a source signal in parallel over multiple paths in a network. In particular, a source client signal or data signal is demultiplexed into multiple parallel subchannels or subsignals, of lower data rates to be transmitted over a fiber. A receiver will then receive the parallel subsignals and reconstruct the source client signal at its output. Because the receiver must receive all the subsignals in order to recreate the source signal, the timing of the transmission and reception of the subsignals at the receiver is important.
Skew is one factor that is important in the timing of the transmission and reception of subsignals at the receiver. Skew may be defined as a variation relative to the initial timing of a component of a launched data signal. Skew can arise from many different causes depending upon the transmission medium over which information is communication. For example, intrachannel skew and interchannel skew can arise because different wavelength carriers propagate at different rates. In particular, a high frequency carrier signal will generally take a longer period of time to propagate along an identical length fiber as a lower frequency carrier signal.
Intrachannel skew and interchannel skew may also be generated by wavelength-dependent interactions between an optical signal and an optical fiber. In particular, the optical fiber may skew carriers differently depending on the wavelength(s) thereof. Optical fiber may have variations in refractive index, polarization mode properties, dispersion and chromatic properties, etc. These variations may generate excessive signal skew at the receiver, which translates into a delay of the source signal being reconstructed at the receiver.
Many solutions have been proposed to compensate for the skew variation between multiple parallel communication paths. One solution is to simply provide a buffer of sufficient size that allows proper re-sequencing of data in spite of a large amount of skew. In worst case scenarios, the buffer is able to provide sufficient memory that allows data to be maintained while waiting for traffic to arrive on slower paths. As optical data rates increase, the size and cost of these sufficiently larger buffers also increases.
C. Definition of Terms
Definitions of certain terms are provided below. However, other terms found within the present application may be defined within other sections or known to one of skill in the art. If used throughout this description and the drawings, the following short terms have the following meanings unless otherwise stated:
3R—Optical signal regeneration that includes signal retiming as well as signal reshaping as well as regeneration or re-amplification.
4R—Any electronic reconditioning to correct for transmission impairments other than 3R processing, such as, but not limited to, FEC encoding, decoding and re-encoding.
A/D—Add/Drop.
BMM—Band MUX Module which optically combines multiple banded wavelengths or optical signal groups together
CD—Chromatic Dispersion.
Client Signals are the transmission of any form of data or information as a modulated signal over a network, whether electrically or optically.
Communication Paths include any path over which a client information signal may be transmitted, including different carrier wavelengths, optical carriers, channels, physical paths, fibers or lines, nodes, speed and data rates, transmission medium or type of transmission, skew compensating devices, and modulation.
CWDM—Coarse Wavelength Division Multiplexing—transmission of data on more than one wavelength in a given direction on a grid of spacing greater than 200 GHz.
DCE—Dispersion Compensating Elements either for CD, such as DCFs, dispersion managed solutions, frequency guiding filtering, chirped fiber Bragg gratings or dispersion slope compensation, or for PMD, such as through optical device design to achieve polarization insensitivity or equalization optical circuitry to provide distorted optical signal polarization transformation.
DCF—Dispersion Compensating Fiber.
DEMUX—Demultiplexer.
Digital OEO REGEN—an OEO REGEN that provides digital handling of channel signals including their regeneration in a digital optical network without deployment or need for analog optical components including optical fiber amplifiers.
DWDM—Dense Wavelength Division Multiplexing—transmission of data on more than one wavelength in a given direction on a grid of spacing less than or equal to 200 GHz.
EO—Electrical to Optical signal conversion (from the electrical domain into the optical domain).
LR—Long Reach.
MUX—Multiplexer.
NE—Network Element which includes network nodes or other devices located within a network.
OADM—Optical Add Drop Multiplexer.
OCG—Optical Carrier Group is a grouping of a plurality of optical carrier wavelengths.
OE—Optical to Electrical signal conversion (from the optical domain into the electrical domain).
OEO—Optical to Electrical to Optical signal conversion (from the optical domain into the electrical domain with electrical signal regeneration and then converted back into optical domain) and also sometimes referred to as SONET regenerators.
OEO REGEN—OEO signal REGEN is OEO conversion wherein the signal is regenerated in the electronic domain between conversions to the optical domain. SONET regenerators are one example of OEO REGEN but it is not limited to such regenerators.
OO—Optical to Optical signal conversion (receiving in the optical domain, processing in the optical domain, and transmitting in the optical domain, without conversion of the signal to the electrical domain).
OSC—Optical supervisory channel; a physical carrier outside of the amplifier band that provides transport of the OTM Overhead Signal.
PIC—Photonic Integrated Circuit.
Rx—Receiver, here in reference to optical channel receivers.
RxPIC—Receiver Photonic Integrated Circuit.
SDM—Space Division Multiplexing.
Signal regeneration (regenerating)—Also, may be referred to as signal rejuvenation. This may entail 1R, 2R, 3R or 4R and in a broader sense signal A/D multiplexing, switching, routing, grooming, wavelength conversion as discussed, for example, in the book entitled, “Optical Networks” by Rajiv Ramaswami and Kumar N. Sivarajan, Second Edition, Morgan Kaufmann Publishers, 2002.
Skew Adjustment includes any modification of a skew of a data signal, including preskew, skew correction, skew reduction, skew matching, skew modification, or skew rehabilitation.
SR—Short Reach.
TDM—Time Division Multiplexing.
Tx—Transmitter, here in reference to optical channel transmitters.
TxPIC—Transmitter Photonic Integrated Circuit.
VSW—Virtual Super Wavelength—(a.k.a. super lambda)—a group of co-routed channels whose associated data comprise the complete client signal and whose data is needed at the egress from the network to reconstruct the client signal.
VWG—Virtual Wavelength group—one or more co-routed channels forming a portion of a VSW
WDM—Wavelength Division Multiplexing—transmission of data on more than one wavelength in a given direction.